Field of the Invention
The field of this invention relates to solid state photodetectors, and in particular, a traveling-wave photodetector having an electrically distributed electrode structure.
Photodetectors convert modulated light into electrical signals. Most solid-state photodetectors operate by incident light or photons being absorbed to "create" carriers, that is, electrons and holes. In photoconductors, these carriers change the conductivity of the devices. In devices such as PIN diodes and avalanche photodiodes (APDs), the carriers are swept away to electrical contacts to produce a photocurrent. In either case, a detectable electrical change occurs and light detection is achieved.
A PIN diode is formed when p-type and n-type semiconductor regions are separated by a layer of intrinsic semiconductor material. Intrinsic semiconductor material ideally is pure semiconductor material with no doping, but in reality is semiconductor material that is slightly doped (&lt;10.sup.14 atoms/cm.sup.3).
One measure of the quality of the conversion of light to electrical signal within a photodetector is the quantum efficiency. The quantum efficiency is the ratio of carriers generated to incident photons. That is, if for every photon incident on the device one electron-hole pair is generated, the detector is said to have a quantum efficiency of 100%. Low-bandwidth photodetectors with quantum efficiencies as high as 80% are not uncommon.
Traditional photodetectors are electrically lumped devices. That is, they can be considered to exist at one point in space where all of the photocurrent generation takes place. This means that the bandwidth of traditional photodetectors is generally limited by the resistance and capacitance of the detector. That is, the bandwidth of the photodetector is simply f.sub.-3dB =1/2.pi.RC, where C is the capacitance of the detector and R is the Thevenin equivalent resistance that the detector drives. In order to increase the bandwidth of traditional photodetectors, the RC product must be decreased. Since R is generally fixed by the external circuit driven by the photodetector, the only way to improve bandwidth is to decrease the capacitance C.
The capacitance C is easily decreased by either reducing the surface area or by increasing the electrode spacing of the device. To reduce the surface area, the device must be made smaller. To increase the electrode spacing, the depletion layer in the device must be made thicker. Unfortunately, these changes reduce both the quantum efficiency and power dissipation capabilities of the photodetector.
If the surface area is decreased, the quantum efficiency is reduced because of the practical difficulty in getting all of the light to be absorbed within the small detector. Additionally, an increase in electrode spacing introduces a transit-time frequency response limitation which reduces effective quantum efficiency at high frequencies. In general, increasing the electrical bandwidth yields a reduction in quantum efficiency. Furthermore, the maximum power dissipation of the detector also reduces with decreased detector size.